Further histological evaluation of two additional long-term studies in rats, following single intratracheal instillation of 20 mg Min-U-Sil quartz or of 20 mg hydrofluoric-acid-etched Min-U-Sil quartz, showed high levels of induction of peripheral lung carcinomas, as in the previously reported experiment with a single dose of 12 mg Min-U-Sil quartz. In all three experiments the same histogenetic pattern was observed for epithelial lesions adjacent to silicotic granulomas: marked hyperplasia of the alveolar type II epithelia, adenomatoid proliferation and later development of lung tumors reaching incidences of 85-100 percent in the rats surviving over 17 months. The current results indicate that the 12 mg dose is already producing near-maximal carcinogenic effects. Preliminary histological observations of human lungs with silicosis and lung cancer have shown some remarkable similarities with two of the lesions observed in the experimental silica-treated rats, namely, (a) adenocarcinoma growing among fibrotic tissue and around it ("scar cancer"); and (b) epithelial hyperplasia and proliferative lesions adjacent to silicotic granulomas, in areas distant from the carcinoma.